There are so many variables that I couldn't give you an actual value. You will need to factor in the finish work, style, and design of the basement. A good rule of thumb is that the cost will be about $25 - 30/square foot, plus $4,000 - 7,500 for a bathroom, and that you will be able to recoup about 65% - 75% of what you put into it for resale. The pricing assumes you using a contractor for all the work, it will be less if you do the work yourself (if you're qualified).
Best of luck, and I would recommend you bringing your Realtor into your home to take a look and give you some idea of what the costs and returns will be.